Various forms of chrysotile asbestos (short range, intermediate range, and Canadian Standard B) and glass microfiber (JM-100) were given to male and female rats in an inhalation chamber 7 hours/day, 5 days/week for 12 months. The rats were then held for life time observation. Groups of animals were killed at 3 months and 12 months exposure and 12 months postexposure to evaluate lung lesions. All forms of asbestos (especially the longer fibers) caused fibrosis in the area of the terminal bronchioles which progressed during exposure but remained relatively static during the postexposure period. The glass wool did not produce this lesion. Treatment related pulmonary carcinomas were observed in rats exposed to asbestos, with more being observed with the intermediate range and chrysotile B than with the short range. No treatment related neoplasms were observed with JM-100.